I think that the most important first step in responding to potential terrorist activity is to secure the area. This might mean ensuring that suspects cannot leave and there is a way to differentiate them from victims, but also ensuring that there is no more danger. Immediately evacuating a building, for example, is a good move but you have to ensure that you can contain the occupants in order to determine if any of them are involved in the threat.
Key facts would include the extent of the damage, whether the attack was still ongoing, whether additional risks were created (like ruptured gas lines, collapsing buildings, etc.) I think that initially, the criteria used to gauge the severity would be the number of people injured or killed. The likely impact of mistakes, unfortunately would be that more people would die or that rescue personnel would walk into a situation that placed their lives in jeopardy unnecessarily.